1. Technical Field
This invention concerns the detection of gas particularly but not necessarily exclusively the detection of toxic, flammable or otherwise hazardous gas in a work area such as a petrochemical facility.
2. Related Art
A well known way of detecting a gas is by means of laser diode spectroscopy (LDS). The output wavelength of a laser diode depends upon the applied drive current, so by selectively varying the drive current the output wavelength can be made to scan over an optical absorption line of a target gas to be detected or measured. If the output from such a laser diode is transmitted through a gas sample and focussed onto a suitable optical detector, the signal received by the detector corresponds to the product of the laser's output waveform and the transmission spectrum of the gas sample being illuminated. Then the quantity of target gas in the transmission path through the sample can be determined by measuring the change in received intensity when the laser's wavelength corresponds to the wavelength of the optical absorption line of the target gas. The amount of absorption produced by a given quantity of gas can be determined using Beer's law, I=I0.e−σn, where I is the received intensity at the detector, I0 is the intensity incident upon the sample being illuminated, σ is the absorption cross section of the target gas at the absorption wavelength and n is the total number of target gas molecules in the measurement path.
LDS is widely used in applications requiring high sensitivity—in instrumentation for chemical processes, for instance, and in monitoring atmospheric pollutants. Hitherto, however, LDS has been little used in safety-related applications. The main reason for this is that such applications demand an extremely high level of reliability and in particular very low false alarm rates. Conventional LDS is sufficiently reliable to detect fractional absorbances as low as 1×10−5 in process control or atmospheric monitoring applications, but such small fractional absorbances cannot be detected with an acceptably low false alarm rate for safety applications. In short, the probability of false alarms is considered too high for LDS to be used for applications such as the detection of toxic and flammable gases for which it would otherwise be suitable.